Large-Scale Synthesis of Aluminum Diboride Nanowires by Ni(NO3)2 Catalyst Q.H

Journal of Crystal Growth 346 (2012) 75–78

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Journal of Crystal Growth

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Large-scale synthesis of aluminum diboride nanowires by Ni(NO3)2 catalyst Q.H. Fan a, Y.M. Zhao a,b,n, J. Huang a, L.S. Ouyang a, Q. Kuang a a School of Physics, South China University of Technology, Guangzhou 510640, PR China b State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, PR China article info abstract

Article history: Large-scale aluminum diboride (AlB2) nanowires have been successfully fabricated for the ﬁrst time Received 24 November 2011 using the facile catalysis method with aluminum (Al) powders and boron trichloride (BCl3) gas mixed Received in revised form with hydrogen and argon and Ni(NO3)2 as the catalyst. X-ray diffraction (XRD), scanning electron 2 February 2012 microscopy (SEM), transmission electron microscopy (TEM), and high resolution transmission electron Accepted 14 February 2012 microscopy (HRTEM) were used to characterize the morphologies and structures of the samples. Our Communicated by J.M. Redwing Available online 23 February 2012 results show that the AlB2 nanowires are single crystal. & 2012 Elsevier B.V. All rights reserved. Keywords: A1. Nanostructures A1. Nanowires

B1. AlB2 B2. Facile catalysis method

1. Introduction crystal platelets of AlB2 have been prepared by Sirtl and Woerner [16], and electrical measurements indicate that AlB2 exhibits Since the discovery of carbon nanotubes [1], one-dimensional metallic conduction and the superconductivity was not observed nanostructures have found unique application in electronics [2], down to 1.5 K. Loa et al. [17] studied the crystal structure of AlB2 optoelectronics [3], and catalysts due to their high surface-to- up to 40 GPa by X-ray powder diffraction. In the pressure range volume ratio, enhanced materials characteristics due to the they did not observe a structural phase transition. Recently, the quantum confinement effects, and the high fraction of chemically physical and chemical properties of AlB2 have gained considerable similar surface sites. Meanwhile, the nanostructures of rare-earth interest because the compound is considered as a reaction partner hexaborides and alkaline earth borides are also attractive as an in a hydrogen storage system which has the potential to rever- interesting class of compounds. Zhao and his team [4–10] have sibly desorb and absorb 8.6 wt.% H2 [18–20]. synthesized a series of the rare-earth hexaborides nanostructures The preparation of AlB2 was first reported by Funk et al. [21] and alkaline earth borides nanostructures, such as LaB6, SmB6, by heating a aluminum-rich aluminum–boron mixture at a NdB6, PrB6, EuB6, CeB6 and CaB6. temperature of about 1000 1C. Then, in 1930, Steele and Mills In the borides system, Al–B master alloys are widely used in [22] reported the preparation of an aluminum boride by the the production of electrical conductive grade aluminum to fusion of finely divided Al and powdered B2O3 in an iron crucible. remove transition metal impurities, such as titanium, vanadium, Recently, Wang [23] reported that AlB2 in an Al–B master alloy chromium and zirconium [11,12]. Al–B master alloys are also was produced via chemical reactions of KBF4 and aluminum used in the in situ fabrication of aluminum matrix composites. at 850 1C.

One example is the in-situ fabrication of AlB2 ﬁber reinforced To our knowledge, no results on the preparation and proper- aluminum metal matrix composites using an Al–B master alloy ties of AlB2 nanowires have been reported yet. We herein report [13]. The AlB2 structure type and derivatives thereof are among for the ﬁrst time the characterization of the structural properties the most frequently occurring ones for intermetallic binary and of large-scale single-crystalline AlB2 nanowires. The method ternary compounds [14]. Recently, the discovery of superconduc- employed to synthesize AlB2 nanowires was by the reaction of tivity at Tc 39 K in MgB2 [15], which also crystallizes in the aluminum powders with BCl3 gas on Si substrates using Ni(NO3)2 AlB2 structure, initiated a strong interest in s–p diborides. Single as catalysts. It is expected that AlB2 nanowires can exhibit higher reinforce in the in-situ fabrication of AlB2 reinforced aluminum metal matrix composites and better reaction partner in a hydro-

n gen storage system as well. In addition to the relevant importance Corresponding author at: School of Physics, South China University of Technology, Guangzhou 510640, PR China. Fax: þ86 028 551 1266. with respect to applications, AlB2 nanowires would provide E-mail address: [email protected] (Y.M. Zhao). valuable information in the quest of crystal chemical and physical

0022-0248/$ - see front matter & 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2012.02.026 76 Q.H. Fan et al. / Journal of Crystal Growth 346 (2012) 75–78 properties by experimental as well as theoretical methods, identified by powder X-ray diffraction (XRD, TD 3500). The general including lattice dynamics properties and electron–phonon and morphology of the sample was characterized by a field-emission optical properties due to its quantum confinement effects. scanning electron microscope (SEM, Navo NanoSEM430). The microstructure was studied with a transmission electron microscope (TEM, JEM-2010HR).

2. Experimental 3. Results and discussion The method employed to synthesize AlB2 nanowires was based on the following chemical reaction: The phase identification of the products was carried out by an X-ray powder diffractometer. A step scan mode was adopted with Al (s)þ2BCl (g)þ3H (g)¼AlB (s)þ6HCl (g) 3 2 2 a scanning step of 0.021 and a sampling time of 2 s. Fig. 1(a) shows The reaction was carried out in a quartz tube furnace. The the X-ray diffraction patterns of the as-prepared products. synthetic route can be described as follows: firstly, Ni(NO3)2 6H2O No impurity phase was detected under the resolution of our powders with 8.9 g in weight were dissolved in 30 ml of distilled X-ray diffractometer and all the reflection peaks can be indexed water to get a green transparent solution. A clean silicon substrate as a hexagonal phase of AlB2 (JCPDS card no. 39-1483) with the (about 1 1 cm in size, (100) surface) was immersed in the space group P6/mmm and lattice parameters of a¼0.3009 nm solution for a few seconds, removed and heated at 90 1C. The and c¼0.3262 nm. As shown in Fig. 1(b), the intensities of X-ray

Ni(NO3)2-coating Si substrate with Al powders (purity of 99.9%) on diffraction peaks decreased signiﬁcantly after air exposure for it was loaded in a quartz boat, and the boat was placed in the more than one month. The poisoning in moisture suggests that center of the quartz tube. After the quartz tube was heated to AlB2 samples prepared here are unstable in air. an expected furnace temperature (750 1C) under a mixed gas General overviews of the morphology of the products are

(30% H2þ70% Ar) ﬂow of about 150 ml/min in rate, a steady BCl3 provided by SEM images shown in Fig. 2(a)–(c), where the ﬂow of 30 ml/min was introduced into the quartz tube and nanowires obtained on the Si substrate are large scale and along maintained for 30 mins. After the above procedure the quartz tube the longitudinal direction in shape. This means that we can get was cooled down to the room temperature under the mixed gas the nanowires in a relatively uniform orientation by using this

(30% H2þ70% Ar) atmosphere and then a gray-black layer was experimental procedure. As shown in Fig. 2(c), AlB2 nanowires found on the silicon substrate. The phase of the products was have lateral dimensions about 200 nm, and lengths extending to

Fig. 1. X-ray diffraction patterns of AlB2 obtained from Si substrate: (a) as-prepared and (b) air-exposed for one month.

Fig. 2. SEM images of the AlB2 nanowires: (a) general overviews of the morphology of the products; (b) and (c) enlarged views of the AlB2 nanowires. Q.H. Fan et al. / Journal of Crystal Growth 346 (2012) 75–78 77 more than a few micrometers. Most nanostructures have a electron diffraction spots, corresponding to the hexagonal system smooth surface, and are straight along their axes. with P6/mmm symmetry of a¼0.3009 nm and c¼0.3262 nm In order to understand the mechanism of structure and growth (Fig. 3(c)), but there is also evidence of additional spots which of the AlB2 nanowires, the morphology and microstructure of the cannot be recognized. The extra spots observed may be due to the product were further characterized by TEM. For electron diffrac- partial decomposition of AlB2 under 100-kV beam intensity. tion and microscopy observations, the sample was ﬁrst dispersed To elucidate more details of the structural characteristics of the in ethanol and then collected on a copper grid coated with a thin AlB2 nanowires high-resolution transmission electron microscopy holey carbon ﬁlm. (HRTEM) was also employed. As shown in Fig. 3(d), the lattice

Fig. 3(a) shows the typical TEM images of an individual AlB2 fringes are not uniform which can only be observed locally in nanowire for the as-prepared product at a low magniﬁcation, some regions. The area without the lattice fringes shown in where the smooth surface and uniform diameter along the entire Fig. 3(d) may have resulted from the partial decomposition of nanowire can be observed. AlB2 within the selected area of single nanowire under beam Fig. 3(b) is the selected area electron diffraction (SAED) image intensity. In order to get the clear fringe spacings, the area of the of as-prepared specimens under an accelerating voltage of 200 kV. rectangle in Fig. 3(d) was magniﬁed. The HRTEM images of the

It is surprising that four diffuse, broad concentric rings typical of as-prepared AlB2 nanowires (inset of Fig. 3(d)) show about amorphous material can be observed, which contradict with our 0.2402 nm lattice fringes, which correspond to the [101] planes

X-ray diffraction results (Fig. 1). Considering that the temperature of a hexagonal phase of AlB2 in the X-ray diffraction patterns rise or melting when exposure under 200 kV beam intensity may (Fig. 1(a)) and electron diffraction patterns (Fig. 3(b)). As com- result in the deterioration of the crystallization of AlB2 nanowires, pared with Fig. 3(a), the TEM images of the specimens after air a low beam intensity was adopted, and the evidence of single exposure for more than one month show that the smooth surface crystal of AlB2 is readily observable as reasonably sharply deﬁned, of an individual nanowire (Fig. 3(a)) has been deteriorated bright diffracting spots against the halo pattern produced at seriously along the entire nanowire due to the poisoning in 200 kV beam intensity, as shown in Fig. 3(b). In Fig. 3(c), image moisture. Furthermore, the poisoning in moisture and/or the of as-prepared specimens with decreasing amounts of accelerat- deterioration of the crystallization under beam intensity is also ing voltage of 100-kV electrons not only results in deﬁnite evidenced by HRTEM image as shown in Fig. 3(f), where the lattice fringes corresponding to the [101] planes of as-prepared

AlB2 nanowires (Fig. 3(d)) cannot be observed even in the entire area of the nanowires. As shown in Fig. 1(b), the hexagonal phase with the space group P6/mmm can be documented by X-ray diffraction result for the specimens after air exposure for more than one month although the intensities of X-ray diffraction peaks, when compared with that of the as-prepared specimens, decreased signiﬁcantly. EDS technique was used to identify the chemical composition of nanowires. Fig. 4 is a representative EDS spectrum recorded from the nanowire, and the results show that, besides the B and Al components, there is little amount of Si, and Cu. Because the samples used to analyze the compositions were taken from thin ﬁlm of nanowires, we think that there may be contamination of

our samples with Si substrate (due to etching by BCl3), especially at the positions that contacted with Si substrate. For the copper composition, it should come from the Cu grid. It is difficult to detect the B component accurately because of the small atomic number of light elements, and thus based on the analysis of EDS spectrum, it is difficult to confirm the exact compositions of the

nanostructures with AlB2 or not. Furthermore, as evidenced in Fig. 3(b)–(c), it is also difﬁcult to get the reasonable molar ratio of

Al and B due to the partial decomposition of AlB2 under electron beam intensity when EDS spectrum was recorded. The method

Fig. 3. (a)–(d) As-prepared AlB2 nanowires ((a)TEM images, (b) and (c) electron diffraction under 200 KV and 100 KV voltage, respectively, (d) HRTEM image (inset shows the enlarged HRTEM)); (e)–(f) AlB2 nanowires after air exposure for more than one month, ((e) TEM image, (f) HRTEM (inset shows the enlarged HRTEM)). Fig. 4. A representative EDS spectrum. 78 Q.H. Fan et al. / Journal of Crystal Growth 346 (2012) 75–78

employed to synthesize AlB2 nanowires here was by the reaction Acknowledgments of aluminum powders with BCl3 gas on Si substrates using Ni(NO3)2 as catalyst. It is believed that, based on our experimental This work was funded by NSFC Grant (nos. 50972046 and procedure, the vapor–liquid–solid (VLS) growth mechanism 51172077) supported through NSFC Committee of China and a should be adopted here where the Ni(NO3)2 was used as the Foundation Grant (no. S2011020000521) supported through the catalyst. As demonstrated in Fig. 2(c), a cap at the tip of nanowire Science and Technology Bureau of Guangdong Government. The was also seen, which can be a clue of the conventional vapor– authors are indebted to Dr. Chaolun Liang of the Sun Yat-Sen liquid–solid (VLS) mechanism proposed for nanofibers grown by a University for his assistance with the HRTEM experiments. catalyst-assisted process, in which a transition metal particle is capped at the tip of the fiber and serves as the active catalytic site References [24]. EDS technique was also adopted to study the chemical composition of the cap at the tip of the nanowire, and the failure [1] S. Iijima, Nature 354 (1991) 56. of our effort to detect Ni component on the tip of AlB2 nanowires [2] Y. Cui, C.M. Lieber, Science (New York, NY) 291 (2001) 851. [3] A.P. Alivisatos, Science (New York, NY) 271 (1996) 933. may be due to the partial ablation of AlB2 under electron beam [4] J.Q. Xu, Y.M. Zhao, C.Y. Zou, Chemical Physics Letters 423 (2006) 138. intensity when EDS spectrum was recorded. Thus, the growth [5] J.Q. Xu, Y.M. Zhao, Z.D. Shi, C.Y. Zou, Q.W. Ding, Journal of Crystal Growth 310 mechanism dominated by the AlB2 nanowires growing process (2008) 3443. under this study is still open now and a more detail study will be [6] Q.W. Ding, Y.M. Zhao, J.Q. Xu, C.Y. Zou, Solid State Communications 141 (2007) 53. carried out in the future works. [7] J.Q. Xu, X.L. Chen, Y.M. Zhao, C.Y. Zou, Q.W. Ding, Nanotechnology 18 (2007) 115621. [8] J.Q. Xu, X.L. Chen, Y.M. Zhao, C.Y. Zou, Q.W. Ding, J.K. Jian, Journal of Crystal Growth 303 (2007) 466. 4. Conclusions [9] C.Y. Zou, Y.M. Zhao, J.Q. Xu, Journal of Crystal Growth 291 (2006) 112. [10] J.Q. Xu, Y.M. Zhao, C.Y. Zou, Q.W. Ding, Journal of Solid State Chemistry 180 (2007) 2577. In summary, large-scale AlB2 nanowires have been fabricated [11] W. Stiller, T. Ingenlath, Aluminium 60 (1984) E577. for the first time using aluminum (Al) powders and boron [12] P.S. Cooper, M.A. Kearns, Materials Science Forum 217–222 (1996) 141. trichloride (BCl3) gas mixed with hydrogen and argon, where [13] C. Deppisch, G. Liu, J.K. Shang, J. Economy, Materials Science and Engineering A 225 (1997) 153. the Ni(NO3)2 was used as catalyst. Our experimental results show [14] P. Villars, L.D. Calvert, Persons’s Handbook of Crystallographic Data for that we are able to control the growth rate of the nanowires Intermetallic Phases, second edition, American Society for Metals, Materials exactly, but there are some major factors influencing the growth Park, Ohio, 1997. rate: reaction temperature and flow rate of reactant (BCl3 gas). [15] J. Nagamatsu, N. Nakagawa, T. Muranaka, Y. Zenitani, J. Akimitsu, Nature 410 Generally, the higher the reaction temperature or the higher the (2001) 63. [16] E. Sirtl, L.M. Woerner, Journal of Crystal Growth 16 (3) (1972) 215. flow rate of reactant adopted, the longer and the thicker are the [17] I. Loa, K. Kunc, K. Syassen, P. Bouvier, Physical Review B 66 (2002) 134101. nanowires. The nanowires obtained here have lateral dimensions [18] S. Jin, J. Shim, Y. Cho, K. Yi, O. Zabara, M. Fichtner, Scripta Materialia 58 (2008) about 200 nm, and lengths extending to more than a few micro- 963. [19] X.D. Kang, P. Wang, L.P. Ma, H. Cheng, Applied Physics A 89 (2007) 963. meters. The X-ray diffraction, HRTEM results as well as the SAED [20] D.J. Siegel, C. Wolverton, V. Ozolins, Physical Review B 76 (2007) 134102. images at different accelerating voltages reveal that the AlB2 [21] H. Funk, Zeitschrift fur Anorganische und Allgemeine Chemie 142 (1925) nanowires obtained in our experiments adopt single crystal 269. [22] B.D. Steele, J.E. Mills, Journal of the Chemical Society (1930) 74. structure, and it is unstable in air due to the poisoning in moisture [23] X.M. Wang, Journal of Alloys and Compounds 403 (2005) 283. as well as under electron beam intensity. [24] R.S. Wagner, W.C. Ellis, Applied Physics Letters 4 (1964) 89.